Numerical simulation method and system for strength degradation of bedded shale

Abstract

The application discloses a numerical simulation method and system for strength degradation of bedded shale, and the method comprises the following steps: preparing a plurality of shale samples, and obtaining attribute parameters of the shale samples; injecting drilling fluid into one end of the plurality of shale samples within a preset time, and measuring the actual depth of the drilling fluid invading the beddings of the shale samples; obtaining the compressive strength of each shale sample after the drilling fluid invades, and fitting the compressive strength with the actual depth to obtain a first fitting relationship; constructing a physical model to simulate the simulation depth of the drilling fluid invading the bedded joints under different invasion times, and obtaining a second fitting relationship between the invasion time and the simulation depth; and performing equivalent substitution on the second fitting relationship and the first fitting relationship to obtain the strength degradation of the shale sample after the drilling fluid invades and the strength degradation degree of the shale sample. The method can effectively simulate the process of the drilling fluid invading the shale beddings in the well.

Description

Technical Field

[0001] This invention relates to the field of oil and gas exploration and development, and in particular to a numerical simulation method and system for the strength degradation of layered shale. Background Technology

[0002] Shale oil and gas are important unconventional alternative resources, accounting for an increasingly significant proportion of oil and gas production. Currently, shale oil and gas well drilling projects face serious wellbore instability problems. Drilling fluid, under pressure differential, intrudes into bedding shale, causing fracture propagation or hydration expansion. This alters the mechanical properties of the surrounding rock, leading to shale strength degradation and severe wellbore collapse. In severe cases, this can cause complex accidents such as stuck pipe, or even wellbore abandonment, resulting in huge economic losses. To ensure wellbore stability during shale oil and gas drilling, it is essential to understand the impact of drilling fluid intrusion along bedding planes on shale strength degradation, determine the collapse pressure and collapse cycle of bedding shale, and thus provide a basis for selecting drilling fluid density and determining control measures.

[0003] Indoor experiments to test the strength of stratified shale under drilling fluid invasion conditions are the main method for determining the strength degradation law of shale, but current experimental methods have the following problems:

[0004] 1. Currently, the main method used to study the process of drilling fluid intrusion into stratified shale is immersion. This method involves immersing the shale in drilling fluid, which then intrudes into the shale in all directions under pressure. However, in actual drilling, the drilling fluid only contacts and intrudes into the shale at one end. Therefore, the pattern of drilling fluid intrusion into shale using the immersion method differs significantly from the actual downhole conditions. Consequently, the shale strength degradation pattern derived from this experiment will inevitably differ significantly from the actual situation.

[0005] 2. To determine the time-dependent strength degradation patterns of shale, extensive experiments are required. However, due to the limited availability and high heterogeneity of shale core samples, it is impossible to obtain enough core samples for strength degradation experiments under different intrusion conditions (pressure and time). Furthermore, the significant differences in core properties at adjacent depths make it impossible to compare strength degradation patterns under identical conditions. Consequently, relying solely on laboratory experiments to determine the strength degradation patterns of shale after drilling fluid intrusion into the formation is inaccurate.

[0006] Therefore, it is necessary to propose a numerical simulation method that can realistically simulate the process of drilling fluid intrusion into shale bedding while reducing the workload of laboratory experiments. This method can solve the problems of limited core samples and strong heterogeneity faced by laboratory experiments, and obtain more accurate laws on the deterioration of shale strength after drilling fluid intrusion. It has important guiding value for the selection of drilling fluid density and the determination of control measures. Summary of the Invention

[0007] The purpose of this invention is to provide a numerical simulation method and system for the strength degradation of layered shale. This method can effectively simulate the process of downhole drilling fluid intruding into shale bedding. Only a small number of laboratory experiments are needed to analyze a large number of shale strength degradation laws through numerical simulation, avoiding the problems of limited core samples and strong heterogeneity in laboratory experimental research.

[0008] To achieve the above objectives, this invention provides a numerical simulation method for the strength degradation of layered shale.

[0009] In one embodiment of the present invention, a numerical simulation method for the strength degradation of layered shale includes:

[0010] Multiple shale samples were prepared, and the property parameters of the shale samples were obtained. The property parameters included the permeability, porosity, and pore size of the rock matrix and bedding fractures.

[0011] Drilling fluid is injected into one end of multiple shale samples within a preset time period, and the actual depth of the drilling fluid penetrating the bedding of the shale samples is measured; wherein, the displacement pressure of the injected drilling fluid is the actual bottom hole pressure difference, which is the difference between the bottom hole wall fluid column pressure and the pore pressure.

[0012] The compressive strength of each shale sample after the intrusion of drilling fluid is obtained, and the compressive strength is fitted with the actual depth to obtain a first fitting relationship;

[0013] Based on the aforementioned property parameters, a bedding shale physical model of the shale sample is constructed to simulate the simulated depth of drilling fluid intrusion into bedding fractures at different intrusion times, and to obtain a second fitting relationship between the intrusion time and the simulated depth.

[0014] Based on the second fitting relationship and the first fitting relationship, the degree of strength degradation of the shale sample is determined. The degree of strength degradation is the relationship between the compressive strength of the shale sample after intrusion into drilling fluid and the intrusion time.

[0015] Preferably, the method includes:

[0016] The property parameters of the shale sample were obtained by CT scanning.

[0017] Preferably, the method further includes:

[0018] The shale sample was saturated with fluorinated oil to ensure that there was no hydrogen-containing liquid in the shale sample.

[0019] Preferably, before injecting drilling fluid, the method further includes: loading the shale sample into a sealed clamping device and applying confining pressure to the clamping device, the confining pressure being greater than the displacement pressure of the drilling fluid.

[0020] Preferably, before injecting drilling fluid, the method further includes: heating the temperature of the clamping device to the temperature of the actual cored formation.

[0021] Preferably, the method includes:

[0022] The actual depth of the drilling fluid intrusion into the bedding of the shale sample was determined by nuclear magnetic resonance imaging.

[0023] Preferably, obtaining the compressive strength of each shale sample after the intrusion of drilling fluid includes:

[0024] The shale sample, after being infiltrated with drilling fluid, is placed inside a high-pressure device;

[0025] Increase the confining pressure within the high-pressure device;

[0026] An axial load is applied to the shale sample inside the high-pressure device;

[0027] The stress and strain of the shale sample were collected and a stress-strain curve was plotted. The maximum stress value in the stress-strain curve is the compressive strength of the shale sample.

[0028] Preferably, the method further includes:

[0029] By changing the displacement pressure, drilling fluid is injected again into one end of the shale sample to obtain the degree of degradation of the shale sample under the changed displacement pressure.

[0030] Preferably, the method further includes:

[0031] The degree of degradation of shale samples after drilling fluid intrusion into the samples with different property parameters was simulated.

[0032] A numerical simulation system for the strength of layered shale, as described in one embodiment of the present invention, includes: a system for implementing the numerical simulation method for the strength of layered shale as described in any of the above claims.

[0033] Compared with existing technologies, the numerical simulation method of the present invention only requires a small number of indoor experiments to obtain the first fitting relationship, and then obtains the second fitting relationship through a physical model. Combining the two, a large number of shale strength degradation laws can be analyzed through numerical simulation, effectively solving the shortcomings of existing indoor experimental methods. Attached Figure Description

[0034] Figure 1 This is a schematic flowchart of a numerical simulation method for the strength degradation of layered shale according to an embodiment of the present invention;

[0035] Figure 2This is a schematic diagram of the process for obtaining the compressive strength of each shale sample after the intrusion drilling fluid, according to an embodiment of the present invention;

[0036] Figure 3 This is a schematic diagram of the structure of a high-voltage device according to an embodiment of the present invention;

[0037] Figure 4 This is a first fitting relationship vector diagram according to an embodiment of the present invention;

[0038] Figure 5 This is a second fitting relationship vector diagram according to an embodiment of the present invention;

[0039] Figure 6 This is a second fitting relationship vector diagram according to another embodiment of the present invention;

[0040] Figure 7 This is a second fitting relationship vector diagram according to another embodiment of the present invention. Detailed Implementation

[0041] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0042] Unless otherwise expressly stated, throughout the specification and claims, the term "comprising" or its variations such as "including" or "comprises" shall be understood to include the stated elements or components without excluding other elements or other components.

[0043] like Figure 1 As shown, a numerical simulation method for the strength degradation of layered shale according to a preferred embodiment of the present invention includes:

[0044] Step 101: Prepare multiple shale samples and obtain their property parameters, including the permeability, porosity, and pore size of the rock matrix and bedding fractures. Specifically, to facilitate subsequent displacement of the shale samples by drilling fluid, prepare multiple cylindrical shale samples with a diameter of 25 mm and a height of 50 mm and polish them. The bedding fracture sizes of the prepared shale samples need to be relatively similar. Due to the presence of bedding fractures in the shale samples, the permeability is set in two ways: rock matrix permeability and bedding fracture permeability. The obtained property parameters of the shale samples can be used as parameters for numerical simulation through a model.

[0045] Step 102: Drilling fluid is injected into one end of multiple shale samples within a preset time period, and the actual depth of the drilling fluid penetration into the shale sample bedding is measured. Since the shale samples are cylindrical, applying confining pressure to the shale samples allows the drilling fluid to penetrate from one end of the cylindrical shale sample. The displacement pressure of the injected drilling fluid is the actual bottom hole pressure differential, which is the difference between the bottom hole wall fluid column pressure and the pore pressure. Specifically, drilling fluid is injected from one end of the shale sample under the displacement pressure of the actual bottom hole pressure differential to ensure that actual working conditions are simulated as closely as possible. In a preferred embodiment, multiple sets of penetration experiments are conducted at different preset times to obtain the penetration conditions of the drilling fluid at different preset times for fitting.

[0046] Step 103: Obtain the compressive strength of each shale sample after the intrusion of drilling fluid, and fit the compressive strength with the actual depth to obtain a first fitting relationship.

[0047] Specifically, after multiple shale samples were penetrated by drilling fluid at a preset time, the actual depth of drilling fluid penetration and the compressive strength of the shale samples at the corresponding actual depth were measured. A vector diagram of the actual depth of drilling fluid penetration and the compressive strength was created, and the data was fitted using plotting software to obtain the first fitting relationship σ between the compressive strength and the actual depth of drilling fluid penetration. c = f(x), where σ c denoted as compressive strength, and x as the actual depth of drilling fluid penetration. The plotting software can be chosen freely, such as Origin, Excel, MATLAB, etc. During the fitting process, various functions can be selected, and the function with the closest fit result, such as linear, logarithmic, exponential, and power functions, is chosen. The power function has the best fit.

[0048] Step 104: Construct a bedding shale physical model of the shale sample based on the attribute parameters to simulate the simulated depth of drilling fluid penetration into bedding fractures at different penetration times, and obtain a second fitting relationship between the penetration time and the simulated depth. This simulated depth is the penetration depth under simulated actual conditions. Specifically, simulate the simulated depth of drilling fluid along bedding fractures at different penetration times under a specified displacement pressure; based on the simulated penetration time and depth data, draw a vector diagram of the simulated penetration depth and penetration time, and perform data fitting using plotting software to obtain the second fitting relationship between the simulated penetration depth and penetration time as x' = f(t), where x' is the simulated penetration depth of the drilling fluid and t is the simulated penetration time.

[0049] In a preferred embodiment, a physical model of layered shale is constructed using COMSOL. The physical fields selected are Darcy flow and fracture flow. The dimensions, porosity, and permeability of the layered shale and its bedding fractures are set, and the injection end and injection end pressure are marked. The dimensions, porosity, and permeability of the layered shale and its bedding fractures are obtained through the attribute parameters in step 101. In actual working conditions, drilling fluid invades the reservoir along the shale bedding fractures. In COMSOL software simulation, Darcy flow can effectively reflect the linear relationship between seepage velocity and hydraulic gradient in porous media, while fracture flow can better simulate the flow of drilling fluid along bedding fractures and the variation of pressure in layered shale.

[0050] Step 105: Substitute the second fitting relationship with the first fitting relationship by an equal amount to obtain the strength degradation degree of the shale sample. The strength degradation degree is the relationship between the compressive strength of the shale sample after drilling fluid intrusion and the intrusion time. Specifically, the first fitting relationship σ obtained in steps 103 and 104 is... c By substituting f(x) with the second fitting relationship x = f(t), we can obtain the relationship between the compressive strength of the shale sample and the intrusion time after drilling fluid intrusion under a specified driving pressure. c =f(t), this relationship can characterize the degree of strength degradation of the shale sample. Specifically, when performing the substitution, the actual depth x in the first fitting relationship and the simulated depth x' in the second fitting relationship are treated as the same variable and substituted. That is, by simultaneously solving the first and second fitting relationships, the relationship between the compressive strength and intrusion time of the shale sample is obtained, which reflects the relationship σ between the compressive strength and intrusion time of the shale sample. c = f(t).

[0051] The numerical simulation method for the strength degradation of layered shale described in a specific embodiment of the present invention preferably includes:

[0052] The property parameters of the shale sample were obtained through CT scanning. The development of the layers was determined based on the CT scan. Specifically, the property characteristics can also be obtained through actual field conditions, or simultaneously based on the CT scan and field conditions.

[0053] The numerical simulation method for the strength degradation of layered shale described in a specific embodiment of the present invention, preferably, further includes:

[0054] The shale sample contained no hydrogen-containing liquid. Specifically, the shale sample was saturated with fluorinated oil to ensure it contained no hydrogen-containing liquid. The fluorinated oil, which contains no hydrogen atoms, was saturated to eliminate the influence of hydrogen-containing liquids other than the infiltrating drilling fluid on the subsequent NMR scan results.

[0055] The numerical simulation method for the strength degradation of layered shale described in a specific embodiment of the present invention preferably includes, before injecting drilling fluid, the method further comprising: loading the shale sample into a sealed clamping device and applying a confining pressure to the clamping device, the confining pressure being greater than the displacement pressure of the drilling fluid. Specifically, the clamping device can be a core holder, and after loading the shale sample into the core holder, a confining pressure is applied, the magnitude of which is 2-3 MPa greater than the displacement pressure.

[0056] The numerical simulation method for the strength degradation of layered shale described in a specific embodiment of the present invention preferably includes, before injecting drilling fluid, the method further comprising: heating the temperature of the clamping device to the temperature of the actual cored formation to better simulate the actual working conditions on site. Specifically, the ambient temperature is preheated to the temperature of the actual cored formation using a constant temperature chamber. After the constant temperature chamber reaches the required temperature, the clamping device containing the shale sample is placed in for the experiment, so that the temperature required for the invasion experiment is set as the temperature of the actual cored formation.

[0057] The numerical simulation method for the strength degradation of layered shale described in a specific embodiment of the present invention preferably includes:

[0058] The actual depth of the drilling fluid penetration into the shale sample bedding was determined using nuclear magnetic resonance imaging (NMR). Specifically, NMR is one method for detecting penetration depth; the position of hydrogen atoms in the NMR image can accurately determine the distance of the drilling fluid penetration as the actual depth of penetration into the shale sample bedding. Preferably, the drilling fluid penetrates simultaneously along the shale matrix and bedding fractures, with deeper penetration along the bedding fractures; therefore, the deepest penetration point is taken as the actual depth of drilling fluid penetration.

[0059] The numerical simulation method for the strength degradation of layered shale described in the specific embodiments of the present invention preferably involves measuring the compressive strength (σ) of the shale sample after drilling fluid intrusion using a triaxial testing apparatus. c Specifically, such as Figure 2 and Figure 3 As shown, the compressive strength of each shale sample obtained after the intrusion of drilling fluid includes:

[0060] Step 201, place the shale sample 41 after the intrusion experiment into the following position: Figure 3 The high-pressure device 40 shown is used within the pressure vessel. Preferably, the high-pressure device 40 can be an autoclave, within which a triaxial experiment is performed to determine the shale strength.

[0061] Step 202: Increase the confining pressure within the high-pressure device. In a specific embodiment, the confining pressure within the high-pressure device 40 is slowly increased to a preset value using a confining pressure pump 42. This preset value is set based on the needs of subsequent experimental research and the actual working conditions on site.

[0062] Step 203: Apply an axial load to the shale sample within the high-pressure device. Specifically, apply an axial load to sample 41 using a hydraulic press. Hydraulic press 43 is used to apply the axial load.

[0063] Step 204: The computer 44 collects the stress and strain of the shale sample and plots a stress-strain curve. The maximum stress value in the stress-strain curve is the compressive strength of the shale sample. Specifically, the data acquisition device collects and records the stress and strain of the rock sample during the application process until the rock sample fails, at which point the axial load is stopped. According to rock mechanics theory, the rock core will release stress after reaching its compressive strength. Therefore, the compressive strength of each shale sample after drilling fluid intrusion can be determined by judging from the triaxial stress-strain curve. When the shale sample fails, the maximum stress value in the stress-strain curve is recorded as the compressive strength of the shale sample after drilling fluid intrusion at this point.

[0064] The numerical simulation method for the strength degradation of layered shale described in a specific embodiment of the present invention, preferably, further includes:

[0065] Drilling fluid is injected into one end of the shale sample using a second displacement pressure to obtain the degree of degradation of the shale sample under the second displacement pressure.

[0066] The numerical simulation method for the strength degradation of layered shale described in a specific embodiment of the present invention, preferably, further includes:

[0067] The degree of degradation of the shale samples after drilling fluid intrusion under different bedding plane conditions was obtained.

[0068] Specifically, by changing the displacement pressure and the development of bedding planes, and repeating steps 101 to 105, the relationship between the compressive strength of shale samples after drilling fluid intrusion and the intrusion time under different displacement pressures and bedding planes can be obtained, which can then be used to evaluate the degree of deterioration of the shale samples.

[0069] This invention also provides a numerical simulation system for the strength of layered shale, comprising: a system for implementing the numerical simulation method for the strength of layered shale as described in any of the above specific embodiments. This system can execute the following specific embodiment:

[0070] Prepare and polish cylindrical shale samples with a diameter of 25 mm and a height of 50 mm.

[0071] Shale samples were placed in a CT scanner to observe the distribution and size of bedding fractures and determine the pore throat to be 0.1 mm.

[0072] The shale sample was saturated with fluorinated oil, loaded into the core holder, and confining pressure was applied and the temperature was raised to the required temperature of 96°C. Drilling fluid was injected from one end of the shale sample under a displacement pressure of 10 MPa.

[0073] One hour after drilling fluid penetrated the shale bedding, the actual depth of the penetration was determined to be 2 mm using nuclear magnetic resonance imaging.

[0074] The compressive strength σ of shale samples after drilling fluid intrusion was determined using a triaxial testing apparatus. c The specific steps include:

[0075] (1) Place the shale sample after drilling fluid intrusion into the autoclave;

[0076] (2) Slowly increase the confining pressure to 10 MPa;

[0077] (3) Turn on the hydraulic press to apply an axial load to the shale sample;

[0078] (4) Use a data acquisition system to record the stress and strain of the shale sample during the application of axial load until the shale sample is destroyed and the application is stopped;

[0079] (5) Record the compressive strength of the shale sample after the drilling fluid penetrated 2 mm at this time as 73.95 MPa.

[0080] Shale samples with similar bedding fracture sizes were selected, and the above steps were repeated to measure the drilling fluid penetration depth and corresponding compressive strength of multiple shale samples at different preset time periods. The data are shown in Table 1. A vector diagram of the first fitted relationship between drilling fluid penetration depth and compressive strength is shown below. Figure 4 As shown, where, Figure 4 The penetration depth in the middle is the actual depth. By using plotting software to fit the data, the fitting relationship between the compressive strength and the actual penetration depth of the drilling fluid is obtained.

[0081] σ c =101.21x -0.434 (1)

[0082] In the formula: σ c — Compressive strength, MPa;

[0083] x — the actual depth of the intrusion, in mm.

[0084] Table 1. Compressive strength of shale at different drilling fluid penetration depths

[0085] Drilling fluid penetration depth x (mm) 2 3 4 6 11 15 <![CDATA[Compressive strength σ c (MPa)]]> 73.95 62.37 55.45 48.13 36.36 30.24

[0086] Based on the development of the strata obtained from CT scans, a physical model of the layered shale was constructed using COMSOL, with the porosity of the layered shale set at 0.05% and the permeability at 10⁻⁶. -4 The drilling fluid was simulated with a displacement pressure of 5 MPa and a pore size of 0.1 mm along the bedding fracture, a porosity of 4%, and a permeability of 0.58 mD. The injection end and injection pressure were also marked. The simulated depth of drilling fluid penetration along the bedding fracture at different times was then calculated. Based on the simulation data of penetration time and depth, a vector diagram of the simulated depth and penetration time was plotted. Figure 5 As shown, data fitting was performed using graphing software.

[0087] Obtain the fitting relationship between the intrusion depth and the simulated intrusion time:

[0088] x' = 1.3431t 0.898 (2)

[0089] In the formula: x' — simulated depth, mm;

[0090] t — intrusion time, h.

[0091] Based on the first fitting relationship obtained from the above steps, combined with the second fitting relationship, and by substituting the two into equal quantities, the relationship between the compressive strength of the shale sample after drilling fluid invasion under a displacement pressure of 5 MPa and the simulated invasion time is obtained:

[0092] σ c =89.05t -0.3897 (3)

[0093] In the formula: σ c — Compressive strength, MPa;

[0094] t — intrusion time, h.

[0095] By changing the displacement pressure to 10 MPa and repeating the above steps, a second fitting relationship between the drilling fluid penetration depth and the simulated penetration time at a displacement pressure of 10 MPa is obtained, as follows: Figure 6 As shown.

[0096] A second fitting relationship was obtained between the intrusion depth and the simulated intrusion time:

[0097] x' = 1.5628t 0.8866 (4)

[0098] By combining the first and second fitting relationships, the relationship between the compressive strength of the shale sample after drilling fluid intrusion and the intrusion time at a displacement pressure of 10 MPa is obtained:

[0099] σ c =83.38t -0.3848(5)

[0100] Change the permeability of the stratification fracture by 10 -2 mD, repeat the above steps to obtain a permeability of 10. -2 The second fitting relationship between drilling fluid invasion depth and invasion time under mD, such as Figure 7 As shown.

[0101] Obtain the fitted relationship between invasion depth and invasion time:

[0102] x' = 1.0434t 0.5729 (6)

[0103] By combining the first and second fitting relationships, the permeability of the bedding fractures (10) can be obtained. -2 Relationship between compressive strength of shale samples after drilling fluid intrusion at mD and intrusion time:

[0104] σ c =99.36t -0.2486 (7)

[0105] In summary, the numerical simulation method and system for the strength degradation of layered shale described in the embodiments of the present invention have the following beneficial effects:

[0106] 1. Compared with traditional immersion tests, a displacement invasion pressure differential can be applied to one side of the shale sample, which is closest to the invasion process of shale under the pressure of the downhole drilling fluid column.

[0107] 2. Only a small number of indoor experiments are needed to conduct a large number of shale strength degradation analysis through numerical simulation, effectively solving the problem of limited rock cores faced by indoor experiments;

[0108] 3. It can more accurately analyze the strength degradation law of shale samples under the same conditions and compare the strength degradation under different conditions, effectively solving the problem of inaccurate strength degradation law caused by the heterogeneity of shale in indoor experiments.

[0109] 4. It can simulate the relationship between the compressive strength of shale and the invasion time under different inclination angles, thus enabling the evaluation of the degree of shale deterioration after drilling fluid invasion under different well inclination angles.

[0110] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0111] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0112] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0113] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0114] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A numerical simulation method for the strength degradation of layered shale, characterized in that, The method includes: Multiple shale samples were prepared, and the property parameters of the shale samples were obtained. The property parameters included the permeability, porosity, and pore size of the rock matrix and bedding fractures. Drilling fluid is injected into one end of multiple shale samples within a preset time period, and the actual depth of the drilling fluid penetrating the bedding of the shale samples is measured; wherein, the displacement pressure of the injected drilling fluid is the actual bottom hole pressure difference, which is the difference between the bottom hole wall fluid column pressure and the pore pressure. The compressive strength of each shale sample after the drilling fluid intrusion is obtained, and the compressive strength is fitted with the actual depth to obtain a first fitting relationship; Based on the aforementioned property parameters, a bedding shale physical model of the shale sample is constructed to simulate the simulated depth of drilling fluid intrusion into bedding fractures at different intrusion times, and to obtain a second fitting relationship between the intrusion time and the simulated depth. Based on the second fitting relationship and the first fitting relationship, the degree of strength degradation of the shale sample is determined. The degree of strength degradation is the relationship between the compressive strength of the shale sample after intrusion into drilling fluid and the intrusion time.

2. The numerical simulation method for strength degradation of layered shale according to claim 1, characterized in that, The method includes: The property parameters of the shale sample were obtained by CT scanning.

3. The numerical simulation method for strength degradation of layered shale according to claim 1, characterized in that, The method further includes: The shale sample was saturated with fluorinated oil to ensure that there was no hydrogen-containing liquid in the shale sample.

4. The numerical simulation method for strength degradation of layered shale according to claim 1, characterized in that, Before injecting drilling fluid, the method further includes: loading the shale sample into a sealed clamping device and applying confining pressure to the clamping device, the confining pressure being greater than the displacement pressure of the drilling fluid.

5. The numerical simulation method for strength degradation of layered shale according to claim 4, characterized in that, Before injecting drilling fluid, the method further includes heating the temperature of the clamping device to the temperature of the actual cored formation.

6. The numerical simulation method for strength degradation of layered shale according to claim 1, characterized in that, The method includes: The actual depth of the drilling fluid intrusion into the bedding of the shale sample was determined by nuclear magnetic resonance imaging.

7. The numerical simulation method for strength degradation of layered shale according to claim 1, characterized in that, The compressive strength of each shale sample obtained after the intrusion drilling fluid includes: The shale sample, after being infiltrated with drilling fluid, is placed inside a high-pressure device; Increase the confining pressure within the high-pressure device; An axial load is applied to the shale sample inside the high-pressure device; The stress and strain of the shale sample were collected and a stress-strain curve was plotted. The maximum stress value in the stress-strain curve is the compressive strength of the shale sample.

8. The numerical simulation method for strength degradation of layered shale according to claim 1, characterized in that, The method further includes: By changing the displacement pressure, drilling fluid is injected again into one end of the shale sample to obtain the degree of degradation of the shale sample under the changed displacement pressure.

9. The numerical simulation method for strength degradation of layered shale according to claim 1, characterized in that, The method further includes: The degree of degradation of shale samples after drilling fluid intrusion into the samples with different property parameters was simulated.

10. A numerical simulation system for the strength degradation of layered shale, characterized in that, include: A system for implementing the numerical simulation method for the strength degradation of layered shale as described in any one of claims 1-9.

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